Thermal-dependent unoccupied electronic structure of a C60 monolayer film adsorbed on a Si(111)-(7×7) surface

We have measured the unoccupied electronic structures of a C 60 monolayer film adsorbed on a Si(1 1 1)-(7×7) surface at 300 and 500 K, using inverse photoemission spectroscopy (IPES) and near edge X-ray absorption fine structure (NEXAFS). At 300 K, both IPES and NEXAFS spectra reveal that most of C 60 molecules interact with the Si surface by van der Waals force at a coverage of 1.0 monolayer. After annealing the sample at 500 K, the full-width at half maximum of the unoccupied molecular orbitals becomes broader indicating the chemisorption at this temperature. Moreover, the NEXAFS spectrum shows the shift of the lowest unoccupied molecular orbital (LUMO) to the higher photon energy side and the decrease in intensity of the LUMO+1. These results suggest that the strong interaction induced at 500 K has a covalent character, to which the LUMO+1 contributes.